This started happening for me yesterday after I installed some Mac OS updates (including a Java-related update). I suspect this is the reason but I have no solution yet.

Rename (or delete) secmod.db (secmod.db.old) and maybe also cert8.db (cert8.db.old) in the Profile Folder in case there is a problem with the files. The file cert8.db stores your user certificates, so if you have user certificates then you may want to export them now and import them after having removed cert8.db. See Tools > Options > Advanced > Encryption: Certificates: View Certificates If that works then you can delete the renamed files or undo the changes if you want to revert the process.

when i deleted the cert8.dat from the profile folder (it is recreated on next start of firefox), this problem was fixed

Deleting cert8.dat did the trick for me. Thanks!

BTW, I was using Chrome in the interim for a few weeks and wasn't a fan. I'm happy to be back to Firefox.

for MAC users u delete those secmod.db and cert8.db files. these files can be found in Library -> application support ->firefox ---> profiles --> xxxx.default ---> then those 2 files can be found there. hav fun with firefox! hope it helps

There is no Firefox folder in my LIbrary under Application support. Where else should I look?

In Mac OS X v10.7, the $HOME/Library folder is a hidden folder.

Open Finder and use one of these:

• Go > Go To Folder (Shift-Command-G) and in the dialog type: ~/Library
• Open the "Go" menu and hold down the Option key to make the Library appear

You can also use this command in a Terminal window to remove the hidden flag.

• Mac HD > Applications > Utilities > Terminal
• chflags nohidden ~/Library

I can find the LIbrary folder with no problem. But there is no Firefox folder in the application support folder, and if it's hidden, I can't make it appear.